1. Field of the Invention
The present invention relates to a communication system using a light guide, and particularly to a sheet light guide which is formed to contain light-scattering particles in a sheet optical medium and propagates light incident from one end surface thereof to the other end surface side while scattering the light by the particles.
2. Description of the Related Art
Heretofore, for example, as described in U.S. Pat. Nos. 5,548,670 and 5,542,017, a light guide has been known, which is formed to contain light-scattering particles in an optical medium such as polymethylmethacrylate (PMMA), and propagates light incident from one end surface thereof to the other end surface side while scattering the light by the particles.
Such a light guide propagates the light while repeatedly scattering the light by the particles in the optical medium in addition to the action of total reflection at an interface between a side end surface of the light guide and a surrounding medium (air or cladding layer). Accordingly, in comparison with a light guide which propagates the light only by the action of the total reflection, the light guide thus described has an advantage in that light, the intensity thereof being made more uniform, can be taken out of an emitting end surface. For example, as described in U.S. Pat. No. 5,822,475, it is conceivable that by utilizing this advantage, such a type of light guide applies to the purpose of constituting an optical data bus (sheet bus). The optical data bus is that which is formed by use of a sheet optical medium, and in which one input unit inputting an optical signal is coupled to one end surface thereof, and a plurality of output units are coupled to the other end surface, thus distributing the inputted optical signal as a common signal to the plurality of output units.
Moreover, for example, as described in Japanese Unexamined Patent Publication No. 11(1999)-31035, as such an optical data bus, one has been also known, in which light-scattering portions are provided on a light-incident-side end portion of the sheet optical medium so as to correspond to respective signal light incident portions, and signal light scattered and branched by the light-scattering portions is made to propagate toward a light-emitting-side end portion of the optical medium.
In a communication system using a sheet light guide formed to contain light-scattering particles in the sheet optical medium as described in U.S. Pat. No. 5,822,475, a requirement exists that a received signal be distributed and outputted in parallel to many lines. In order to respond to such a requirement, it is satisfactory if the number of photodetectors coupled to the sheet light guide is increased. However, when the number of nodes is increased, a problem occurs that an S/N ratio of the signal is lowered according to the increase.